Perpendicularly disposed coils have been mounted on a driverless vehicle and used to detect the electromagnetic field surrounding a guide cable for automatically guiding the driverless vehicle along the cable. In known apparatus, one coil is disposed vertically and the other coil is disposed horizontally. Voltages induced in the coils are compared and used to determine the lateral location of the coils relative to the guide cable. This location information is processed and used to steer the vehicle.
The output voltage associated with these coils varies proportionately with current frequency in the guide cable, guide cable current magnitude, radial distance from the guide cable, coil core size, number of coil wire turns and the angle found between the major axis of the coil relative to a line from the cable to the center of the coil, referred to as angle beta.
As each coil is rotated in a plane perpendicular to the cable generating the electromagnetic field, it's output will become maximum when the coil core is parallel to the circular lines of flux. Its output will become minimum (zero) when the coil core is perpendicular to the flux, i.e. pointing to, or away from, the wire. Thus the relative effectiveness of the coil varies as the `sin` of the angle beta.
The sin (beta) term, reflects the ratio of the radial to circular field sensed at each coil location and affects the sensor output. Therefore, the difference in magnitude sensed between the two coils is based on the radius term, the respective distances between each coil and the cable current. Changes in any of these factors have profound effects on the output signal. Additionally, depending on angle beta, some of either the radial and/or circular field information associated with conventional apparatus must be discarded, resulting in a less than ideal signal to noise ratios. Another disadvantage with conventional coil arrangements is that the information provided by the coils only indicates the approximate lateral displacement from the guide cable and not the measure or distance of the lateral displacement. Therefore, vehicle steering correction can only be made in the direction opposite the displacement and not with precision.